This invention relates to mineral-insulated metal-sheathed electrically conductive cables of a novel integral design and a process for making the same.
The cables of the present invention are suitable for use as thermocouple cables, heat detectors and heating elements that are particularly useful at high temperatures. The invention utilizes nickel-base alloys as sheath materials and thermoelement conductors which have been specifically developed for the purpose. In one aspect the invention provides nickel-based thermocouple cables having enhanced thermomechanical properties, superior oxidation resistance, greater longevity and greater thermoelectric stability under longer time periods and over a range of higher temperatures up to 1300.degree. C., than existing base-metal cables and sensor systems of the same general kind. The invention also provides electrically conductive cables including such cables suitable for use as heat detectors and heating elements.
Nickel-base alloys have been used as thermocouples since the early part of this century. An earlier and now universal nickel-base thermocouple is designated TYPE K by the Instrument Society of America (ISA). A more recent and more stable nickel-base thermocouple is designated TYPE N by the ISA. Typical nominal compositions of the thermoelement alloys which comprise these thermocouple systems are as follows:
______________________________________ ALLOY COMPOSITION (wt. %) (ISA symbol) Cr Mn Al Si Co Mg ______________________________________ KP 9.3 0.4 0.2 NP 14.2 1.4 KN 2.8 2.0 1.0 0.4 NN 4.4 0.2 ______________________________________
The type K thermocouple is recommended to be used in an air atmosphere. At the higher temperatures (above about 1000.degree. C.) this thermocouple fails because of its relatively poor oxidation resistance and hence poor thermoelectric stability. In contrast the type N thermocouple has greatly enhanced oxidation resistance and hence greatly enhanced thermoelectric stability up to about 1300.degree. C.
One way in which attempts have been made to overcome the unstable performance of type K thermocouples at the highest temperatures is to incorporate them in the so-called MIMS (mineral-insulated metal-sheathed) format of a compacted ceramic-insulated thermocouple sensor assembly.
As is well known in the art, a first step in the manufacture of such thermocouple sensors is to produce the so-called MIMS cable which comprises a sheath containing one or more thermoelement conductor wires electrically insulated from the sheath (and from each other when two or more conductor wires are used) by compacted mineral insulation material.